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Rh for something else; he found what he was looking for, and a new planet besides. What he was looking for was one of the so-called nuisances of the heavens, an asteroid, one of about 450, named 433d. To search for it as Herschel had to do, even though its whereabouts was known, called for labour and time. The astronomer, who was on the lookout for it, lessened both by exposing a photographic plate to the starry sky. He was spreading a net to catch planets and comets. A fixed star does not change its place during the exposure of the plate, or, rather, the plate moves as the star moves: a moving body, be it planet or comet, does change its place. A point will thus represent a fixed star; a line, however short, and however faint the trace, represents a moving body. When Herr Witt examined the exposed plate, he saw at once the trace left by the asteroid he was in search of; but another, a fainter and a longer trace of a moving body, was also seen on the plate. It was the trace of a planet hitherto unknown. An examination of the stranger resulted in the discovery that he was a ball twenty miles in diameter, and, excepting our moon, the nearest of the planets to us, so near that he may be made to tell us the exact distance we are from the sun. His discoverer called him Eros, Love or Cupid, evidently from his childish size. Herschel had no such short-cuts to discovery in his day.

An immense impulse was given to the study of the stars by Herschel's discovery. It was not merely what he achieved by being on the spot and on the lookout. It was also by the lesson he taught astronomers to do as he did. A band of twenty-four